Which of the alkali metal is having least melting point?
Atomic size increases as we move down the alkali group. As a result, the binding energies of their atoms in the crystal lattice decrease. Also, the strength of metallic bonds decreases on moving down a group in the periodic table. This causes a decrease in the melting point. Among the given metals, Cs is the largest and has the least melting point.
Size of Cs is the biggest thus, its melting point is the lowest, (d) is correct.